In recreational vehicles it is commmon to have a vent in the roof which is hinged at the front or rear and elevated at the opposite edge by means of an arm or arms connected between the vent cover and the roof structure of the vehicle. The forces on such a vent are significant, considering that the vent is intended to be held open even at highway speeds, and may have an area of three square feet or more. In some installations a single arm is used to brace the vent in its open position, but for larger sizes, a two arm mechanism is required.
A prior art vent operator is described in U.S. Pat. No. 4,068,408 by Peter Hauber. This patent describes the mechanical details of a roof-mounted vent and a two arm vent operator for the reliable adjustment of said vent cover, and is incorporated herein by reference.
The patented device employs a pair of operator arms, each having at their inner ends a sector gear which mates with a common helical gear secured within the operator body. An overlying plate transfers load from the operator body to the recreational vehicle roof. This vent operator, including the flexible operator arms as described therein, combines good mechanical integrity and vibration damping characteristics. However, there is always a need for alternatives of lower cost and improved mechanical characteristics.
Prior art window and vent operating devices are described in U.S. Pat. Nos. 2,801,845, 2,674,452, 2,777,687, 2,774,591, 2,022,036, 3,846,938, 2,699,232, and 2,635,485. In all cases, a helical gear is used to drive the sector arms, but helical gears have several inherent problems. First, a helical gear is difficult to fabricate in that the production of an accurate gear often requires a complex machining process. This is reflected in a higher cost for helical gears. Of course, the gear may be produced by a cheaper method, but then gear accuracy and performance are impaired. Second, spur gears, in comparison to helical gears, have better load resistance characteristics. To use the vent operator apparatus as an example, a wind force felt against the vent is translated into a force felt at the gear assembly as exerted through the sector arms. In a spur gear, this force bears on a flat spur gear tooth surface at a right angle. There is a minimum of slippage. In a helical gear, however, the force is felt at an angle to the tooth surfaces, resulting in lateral forces and slippage.
A third advantage of spur gears over helical or worm gears is the reduced tendency to jam. The forces between spur gear teeth are always at right angles between flat opposed surfaces, maintaining freedom of movement. In worm gears, there is a wedge action resulting in a sliding of teeth surfaces under pressure, resulting in lateral forces and a tendency to jam.
One reason for the limited use of spur gears in vent operators is the greater cost associated with a higher parts count. A number of reduction spur gears would be needed to replace a single helical gear. Since spur gears are usually machined, the added cost of spur gears has prevented their use.
What is needed in the industry is a vent operator comprising spur reduction gears between a driven shaft and the sector arms that can be manufactured economically in commercial quantities.
Additional economies might also be achieved and more attractive assembly result if the additional volume required by the helical gear to achieve speed reduction and mechanical advantage could be accomplished with spur gears.